In the inhalation of drugs in form of aerosols, several factors are of importance for the deposition of the active ingredient in the lung. The deposition of the active ingredient in the lung depends on the particle properties of the active ingredient to be inhaled, such as the particle size, electric charge and hygroscopicity, the inhalation velocity (i.e. respiratory flow) of the patient and the inhalation depth (i.e. tidal volume) of a breath of the patient to be treated.
In various drugs which are to be inhaled in form of aerosols, the amount of inhaled active ingredient has to be given in extremely accurate doses since any overdose could be critical to the patient. In case of conventional inhalation methods, the particle size is adapted to the drug to be administered. However, the patient's breathing pattern is not controlled in any way so that the individual dosage may vary strongly. In case of weak breathing (shallow, rapid respiration), the inhaled drug falls short of the recommended dose, whereas heavy breathing (deep, slow respiration) results in an overdose.
EP-B-0 587 380 describes a drug delivery arrangement that recognizes an inhalation and administers the drug only during an inhalation phase of the breathing cycle while the patient is free to breathe as he likes. This freedom, however, varies from patient to patient, so that the dosages vary considerably. EP-A-0 965 355 describes a controlled inhalator with a predetermined aerosol volume and a limitation of the respiratory flow. In this inhalator, the respiratory flow and the tidal volume are adjusted within certain limits. However, as a mass product, this inhalator cannot be adapted to the concrete requirements as to the pulmonary function of a specific individual. The parameters adjusted for the tidal volume and the respiratory flow are acceptable for the majority of patients, however, the drug administration for the individual patient is not optimal.
Therefore, the following problems occur in practice:                1. Many very obstructive patients are no longer capable of developing the necessary respiratory flow which they would, however, have to develop for an optimal aerosol application;        2. Many of these patients have only very restricted tidal volumes, above all patients with pulmonary emphysema or patients with very small lung volumes;        3. Every patient inhales at a different rate and with a different volume so that the drug dosage within the lung varies widely.        
It is the object of the present invention to provide an improved device for a controlled inhalation of therapeutic aerosols.